A digital memory cell device of this type is used to magnetically store information. An individual memory cell device is generally part of a memory device, often known as an MRAM (magnetic random access memory). Read and/or write operations can be carried out using a memory of this type. Each individual memory cell device comprises a soft-magnetic read and/or write layer systems which is separated by an interlayer from a hard-magnetic reference layer system, which in the present type of memory cell device is designed as an AAF system. The magnetization of the reference layer of the reference layer system is stable and does not change when a field is present, while the magnetization of the soft-magnetic read and/or write layer can be switched by the presence of a field. The two magnetic layer systems may be magnetized parallel or antiparallel to one another. The two abovementioned states in each case represent one bit of information, i.e., a logic zero (“0”) or a logic one (“1”). If the relative orientation of the magnetization of the two layers changes from parallel to antiparallel, or vice versa, the magnetoresistance across this layer structure changes by a few percent. This change in the resistance can be used to read digital information stored in the memory cell. The change in the cell resistance can be detected by a change in voltage. By way of example, in the event of an increase in the voltage, the cell may be occupied with a logic zero (“0”), and in the event of a decrease in the voltages the cell may be occupied with a logic one (“1”). Particularly large resistance changes in the region of a few percent were observed when the magnetization orientation changed from parallel to antiparallel and vice versa in cell structures of the GMR (giant magnetoresistance) type or the TMR (tunnel magnetoresistance) type.
An important advantage of magnetic memory cells of this type is that information is stored permanently and remains stored without any basic power supply being maintained, even when the appliance is switched off, and is immediately available again once the appliance has been switched on, unlike in known conventional semiconductor memories.
A central component is the reference layer system, which is designed as an AAF system (AAF=artificial antiferromagnetic). An AAF system of this type is advantageous on account of its high magnetic rigidity and the relatively low coupling to the measurement layer system as a result of what is known as the orange peel effect and/or as a result of macroscopic magnetostatic coupling fields. An AAF system generally comprises a first magnetic layer or magnetic layer system, an antiferromagnetic coupling layer and a second magnetic layer or magnetic layer system, which is coupled with its magnetization across the antiferromagnetic coupling layer directed oppositely to the magnetization of the lower magnetic layer. An AAF system of this type may, for example, be formed from two magnetic Co layers and an antiferromagnetic coupling layer comprising Cu.
To improve the strength of the AAF system, i.e. its resistance to external fields, it is customary for an antiferromagnetic layer to be arranged on the magnetic layer of the AAF system which is remote from the measurement layer system. Via this antiferromagnetic layer, the magnetization of the directly adjacent magnetic layer is additionally pinned, so that overall the AAF system becomes harder (exchange pinning or exchange biasing).
However, a drawback in this context is the relatively weak coupling between the antiferromagnetic layer and the magnetic layer arranged thereon, which is typically less than 0.3 mJ/m2. A further drawback is that the magnetization of a bias layer system consisting of the antiferromagnetic layer and the AAF system cannot easily be adjusted. This requires the temperature of the bias layer system to be increased above what is known as the blocking temperature of the antiferromagnetic layer, so that the coupling is eliminated, while at the same time a strong external field has to be applied and cooling has to take place in this field. This causes problems in particular for Wheatstone bridge circuits with oppositely oriented AAF systems. Difficulties also occur if the thicknesses of the magnetic layers of the AAF system are approximately identical, since the AAF system then has no net moment or only a minimal net moment and can only be adjusted with difficulty using the external field.
A further drawback of the use of an AAF system with antiferromagnetic layers is that the thickness of the antiferromagnetic layers must be great enough to achieve a sufficiently high blocking temperature. As a result, the distance between the lines which are required for reading and writing, run above and below the layer system and cross one another there, generally known as word and bit lines, increases, which leads to a decrease in the field strength of the energized conductors at the soft-magnetic read and/or write layer system which may have to be switched thereby.